(CNN) - A 20-year-old woman told police she was attacked at an ATM in Pittsburgh by a robber who became angry when he saw a bumper sticker for John McCain on her car, a spokeswoman for the Pittsburgh Police Department said Thursday.

Public Information Officer Diane Richard said police "cannot substantiate" her story, however, and the investigation is ongoing.

Richard said the woman told investigators that a man approached her Wednesday night at an ATM on the city's East End, put a blade to her neck and demanded money. She said she gave him $60 and stepped away from him, Richard said.

But the man "became very angry," the woman said, when he noticed her car had a bumper sticker supporting GOP presidential nominee McCain, according to Richard. The woman said he punched her in the back of the head and knocked her to the ground, where he "continued to punch and kick her while threatening her," the spokeswoman said.

Before he left, the woman said, he carved the letter "B" into her face with a knife, according to Richard. There was no indication what the "B" indicated.

The alleged assailant fled on foot, Richard said.

"We, the police, cannot substantiate this yet," she said. "This is what she told police."

The woman, who is not from Pittsburgh, refused medical attention, Richard said, although she told the investigating officer she would see a doctor Thursday. There was no update on her condition, she said.

Richard said the woman described her alleged attacker as a dark-skinned African-American, 6 feet 4 inches tall with a medium build and short dark hair, wearing dark clothing and shiny shoes.

Richard said investigators were working with the bank to obtain surveillance video of the scene, but added that she did not know if the alleged attack was filmed. She also said that the investigating officer did not write in his report that he saw a McCain sticker on the woman's car. She is awaiting word from the officer on whether he did.

McCain spokeswoman Jill Hazelbaker told CNN that McCain and running mate Sarah Palin "spoke to the victim and her family after learning about the incident earlier this afternoon."

Hazelbaker said the campaign would not offer more detail out of respect for the woman's privacy.

The Obama campaign responded to the report with a statement, saying, "Our thoughts and prayers are with the young woman for her to make a speedy recovery, and we hope that the person who perpetrated this crime is swiftly apprehended and brought to justice."